Apparatus for linear transmitter with improved loop gain stabilization

ABSTRACT

A device for linear transmitter ( 150 ) with improved loop gain stabilization. The linear transmitter ( 150 ) includes a main amplifier loop ( 178 ) and an auxiliary loop ( 176 ). The device includes an adjustable device ( 159 ), connected within the main amplifier feedback loop ( 178 ) and further connected to the auxiliary loop ( 176 ). The adjustable driver ( 159 ) receives an input signal from main amplifier feedback loop ( 178 ), amplifies it in accordance with a gain control signal received from the auxiliary loop ( 176 ) and provides the amplified signal to main amplifier feedback loop ( 178 ).

FIELD OF THE INVENTION

The present invention relates to radio frequency transmitters ingeneral, and to linear radio frequency transmitters having a varyingantenna load, in particular.

BACKGROUND OF THE INVENTION

Radio communication devices use antennas to provide efficienttransmission of radio frequency communication signals. The transmissionportion of a communication device includes a power amplifier foramplifying the radio frequency signals before they are coupled to theantenna for transmission. The power amplifier design often relies onconstant load impedance which is directed at maximizing gain,efficiency, power output level, and the like. The behavior of atransmitter may be affected by its operating environment. For example, atransmitter operating near an electromagnetically reflective structuremay be susceptible to energy reflected back through the antenna into thetransmitter. Reflective energy may be detrimental to transmitterperformance, particularly to the performance of the power amplifier. Anisolator or circulator is often inserted between the antenna and thepower amplifier to protect against changes in load impedance as a resultof reflected energy.

The isolator protects the power amplifier by absorbing the reflectedenergy and preventing it from reaching the amplifier. The isolatordirects the reflected energy to an absorptive load termination. Anisolator typically adds significant cost, size and weight to the designof a radio communication device.

U.S. Pat. No. 5,564,087 to Cygan et al., entitled “Method and apparatusfor a linear transmitter” discloses another solution to the problem ofreflected energy. The solution incorporates a directional coupler todetect the reflected energy and provides a means of adjusting the gainof the power amplifier accordingly. Generally, the gain to the poweramplifier is reduced when high levels of reflected energy are present.In this approach, the circuitry for detection of the reflected energymust operate at the transmission frequency. This adds significant costand complexity to the radio design.

U.S. Pat. No. 5,675,286 to Baker et al., entitled “Method and apparatusfor an improved linear transmission”, is directed to a method andapparatus for isolator elimination, operative at the basebandfrequencies, which is described in detail, herein below. Reference ismade to FIG. 1, which is a schematic illustration of a lineartransmitter block of a radio communication device, generally referenced50, which is known in the art. Transmitter block 50 includes anattenuator 52, three summators 54, 66 and 70, a baseband loop filterunit 56, an up-mixer and radio-frequency (RF) filter unit 58, an RFpower amplifier 60, a down-mixer and baseband filter unit 64, a phaseshifter unit 62, an AGC 68, an adaptor unit 72 and an antenna 74.Summator 54 is connected to attenuator 52, to baseband filter unit 56and to phase shifter unit 62. Summator 66 is connected to basebandfilter unit 56, to up-mixer and RF filter unit 58, to adaptor unit 72and to AGC 68. Summator 70 is connected to AGC 68, to attenuator 52 andto adaptor unit 72. Adaptor 72 is further connected to AGC 68, toattenuator 52 and to phase shifter unit 62. Up-mixer and RF filter unit58 is connected to baseband loop filter unit 56 and to power amplifier60. Down-mixer and baseband filter unit 64 is further connected to poweramplifier 60 and to phase shifter unit 62. Antenna 74 is connected topower amplifier 60 and to down-mixer and baseband filter unit 64.

A signal 80 is provided as input to amplifier feedback loop 78 and toisolator elimination circuit 76. Amplifier feedback loop 78 and isolatorelimination circuit 76 represent the main amplification lop and theauxiliary loop, respectively. Amplifier feedback loop 78 is a closedloop amplifier structure. Typically, this structure can be considered aCartesian feedback loop amplifier. The input signal 80 is generally acomplex digital baseband signal, having quadrature components, i.e.in-phase (I) component and quadrature (Q) component. Signal 80 isprovided to attenuator 52. Attenuator 52 provides an attenuated signalto summator 54. Summator 54 combines this signal with a feedback loopoutput signal 82 and provides a resulting error signal to basebandfilter unit 56. Baseband filter unit 56 provides the filtered errorsignal to up-mixer and RF filter unit 58. Up-mixer and RF filter unit 58up-converts the signal to RF and provides it to power amplifier 60.Power amplifier 60 amplifies the signal and provides the amplifiedsignal to antenna 74 for transmission. Antenna 74 forms a load for poweramplifier 60. It is noted that this load is susceptible to impedancevariations due to its operating environment. Power amplifier 60 providesa portion of the output signal to summator 54, via down-mixer andbaseband circuit 64 and phase shifter unit 62, thereby generatingfeedback loop output signal 82. Feedback loop output signal 82constitutes a feedback signal for controlling the gain of poweramplifier 60 and maintaining transmitter block 50 in the linear mode ofoperation.

Baseband filter unit 56 also provides a filtered error signal 90 tosummator 66. Summator 66 combines error signal 90 with signal 84 fromadaptor 72 and provides the result to AGC 68. AGC 68 constitutes alinear gain control circuit of isolator elimination circuit 76. Adaptor72 controls the gain of AGC 68 by altering the output signal of AGC 68.AGC 68 provides the output signal to summator 70, where it is combinedwith input signal 80. Summator 70 provides the resulting error signal 94to adaptor 72, which produces two output control signals 86 and 88.Control signal 86 adjusts the gain of attenuator 52, and control signal88 adjusts phase shifter 62. Adaptor 72 produces control signals 84, 86and 88 based on input signal 80 and error signal 94.

To avoid the undesirable effects, it is necessary to design atransmitter with sufficient gain and/or power reserve. Such a designwill result in increased complexity, cost, power consumption and more.There is a trade-off between the desired design simplicity andcost-effectiveness on one hand, and necessary dynamic range of systemgain and/or output power, on the other hand. It is desirable to providea linear transmitter, which is cost-effective and simple, yet assuringlinear performance and high signal-to-noise ratio.

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to provide a novel method anddevice for a linear transmitter with improved loop gain stabilization,which alleviates the disadvantages of the prior art.

In accordance with the present invention in a first aspect, there isthus provided a device for minimizing performance degradation of alinear transmitter block of a radio communication device in the presenceof antenna interference, the linear transmitter block including a mainamplifier feedback loop and an auxiliary loop, and an upconverter forconverting baseband signals into radio frequency signals, the devicecomprising an amplifier having an adjustable driver arranged to receivefrom said upconverter an input signal which includes a component fromsaid main amplifier feedback loop and to receive a gain control signalfrom said auxiliary loop, said amplifier being operable to amplify saidreceived input signal according to said gain control signal and toprovide an amplified output signal a portion of which is provided inoperation to said main amplifier feedback loop.

The said upconverter may have an input connection via which a basebandinput signal is in operation applied, the main amplifier feedback loopbeing connected to the input connection. The input connection to theupconverter may include a baseband filter and the main amplifierfeedback loop may be connected to the input connection so that acomponent of a signal applied to the baseband filter is in operationfrom the main amplifier feedback loop.

The said main amplifier loop may include a down-mixer, a baseband filterand a phase shifter.

The said auxiliary loop may include an an AGC (automatic gain control)circuit arranged to receive in operation an portion of a signal which isapplied as an input signal to the upconverter and to apply an outputsignal to the adjustable driver, the auxiliary loop also including anadaptor arranged to control gain of the AGC circuit.

The main amplifier feedback loop may be operable to process a portion ofan output signal of the amplifier by down-converting by said down-mixerand baseband filter unit and phase shifting by said phase shifter. Thephase shifter may be arranged to be controlled by a control signal fromsaid adaptor.

An attenuator may be arranged to provide an output signal which iscombined with an input baseband signal to provide an input to a basebandfilter which in turn is arranged to provide an output signal to theupconverter. The attenuator may be connected to receive in operation aninput signal component from the adaptor, an output signal of theattenuator being combined in operation prior to application to thebaseband filter with an output signal of said main amplifier feedbackloop.

According to the present invention in a second aspect there is provideda linear transmitter bolck including a device according to the firstaspect.

Embodiments of the present invention will now described by way ofexample with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block schematic circuit diagram of a linear transmitterblock of a radio communication device, which is known in the art; and

FIG. 2 is a block schematic circuit diagram of a linear transmitterblock of a radio communication device, constructed and operable inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention overcomes the disadvantages of the prior art byproviding an adjustable driver for improving loop gain stabilization.

Reference is now made to FIG. 2, which is a schematic illustration of alinear transmitter block of a radio communication device, generallyreferenced 150, constructed and operative in accordance with a preferredembodiment of the present invention. Transmitter block 150 includes anattenuator 152, three summators 154, 166 and 170, a baseband loop filterunit 156, an up-mixer and radio-frequency (RF) filter unit 158, anadjustable driver unit 159, a RF power amplifier 160, a down-mixer andbaseband filter unit 164, a phase shifter unit 162, an AGC 168, anadaptor unit 172 and an antenna 174. Summator 154 is connected toattenuator 152, to baseband filter unit 156 and to phase shifter unit162. Summator 166 is connected to baseband filter unit 156, to up-mixerand RF filter unit 158, to adaptor 172 and to AGC 168. Summator 170 isconnected to AGC 168, to attenuator 152 and to adaptor unit 172. Adaptor172 is further connected to AGC 168, to adjustable driver 159, toattenuator 152 and to phase shifter unit 162. Up-mixer and RF filterunit 158 is connected to baseband loop filter unit 156 and to adjustabledriver 159. Power amplifier 160 is connected to adjustable driver 159and to antenna 174. Down-mixer and baseband filter unit 164 is connectedto power amplifier 160 and to phase shifter unit 162. Antenna 174 isconnected to power amplifier 160 and to down-mixer and baseband filterunit 164.

A signal 180 is the input signal to an amplifier feedback loop 178 andan isolator elimination circuit 176. Amplifier feedback loop 178 andisolator elimination circuit 176 represents the main amplification loopand the auxiliary loop, respectively. Amplifier feedback loop 178 is aclosed loop amplifier structure. Typically, this structure can beconsidered a Cartesian feedback loop amplifier. The input signal 180 isgenerally a complex digital baseband signal having quadraturecomponents, i.e. in-phase (I) component and quadrature (Q) component.Signal 180 is provided to attenuator 152. Attenuator 152 provides anattenuated signal to summator 154. Summator 154 combines this signalwith a feedback loop output signal 182 and provides a resulting errorsignal to baseband filter unit 156. Baseband filter unit 156 providesthe filtered error signal to up-mixer and RF filter unit 158. Up-mixerand RF filter unit 158 up-converts the signal to RF and provides it toadjustable driver 159. Adaptor 172 controls the gain of adjustabledriver 159. Adjustable driver 159 provides the output signal to poweramplifier 160. Power amplifier 160 amplifies the signal and provides theamplified signal to antenna 174 for transmission. Antenna 174 forms aload for power amplifier 160. It is noted that this load is susceptibleto impedance variations due to its operating environment. Poweramplifier 160 provides a portion of the output signal to summator 154,via down-mixer and baseband unit 164 and phase shifter unit 162, therebygenerating feedback loop output signal 182. Feedback loop output signal182 constitutes a feedback signal for controlling the gain of poweramplifier 160 and maintaining transmitter block 150 in the linear modeof operation.

Baseband filter unit 156 also provides a filtered error signal 190 tosummator 166. Summator 166 combines error signal 190 with signal 184from adaptor 172 and provides the result to AGC 168. AGC 168 constitutesa linear gain control circuit of isolator elimination circuit 176.Adaptor 172 controls the gain of AGC 168 by altering the output signalof AGC 168. Adaptor 172 controls also the gain of adjustable driver 159.AGC 168 provides the output signal to summator 170, where it is combinedwith input signal 180. Summator 170 provides the resulting error signal194 to adaptor 172, which produces two output control signals 186 and188. Control signals 186 adjusts the gain of attenuator 152, and controlsignal 188 adjusts phase shifter 162. Adaptor 172 produces controlsignals 184, 186 and 188 based on input signal 180 and error signal 194.

In comparing the linear transmitter 150 of the present invention withthat of the prior art, it is significant to note the presence ofadjustable driver 159. Its gain, which is dynamically controlled byadaptor 172, depends on information contained in error signals 190 and194. Since adjustable driver 159 adds additional gain to feedback loop178, it is possible to use low-power isolator elimination circuitry, yetmaintaining required loop gain and linearity of the system 150.

1. A device for minimizing performance degradation of a lineartransmitter block of a radio communication device in the presence ofantenna interference, the linear transmitter block including a mainamplifier closed feedback loop to control gain and linearity of a radiofrequency (RF) power amplifier and an auxiliary loop comprising anisolator eliminator to compensate at baseband for antenna loadvariations on the RF power amplifier, and an upconverter for convertingbaseband signals into radio frequency signals, the device comprising theRF power amplifier having an adjustable driver arranged to receive fromsaid upconverter an input signal which includes a component from saidmain amplifier feedback loop and to receive a gain control signal fromsaid auxiliary loop, the adjustable driver being coupled between theupconverter and the amplifier for causing said amplifier to be operableto amplify said received input signal according to said gain controlsignal and to provide an amplified output signal a portion of which isprovided in operation to said main amplifier feedback loop forgenerating the component included in the input signal from theupconverter.
 2. A device according to claim 1, wherein the upconverterhas an input connection via which a baseband input signal is inoperation applied and the main amplifier feedback loop is connected tothe input connection.
 3. A device according to claim 2, wherein theinput connection to the upconverter includes a baseband filter and themain amplifier feedback loop is connected to the input connection sothat a component of a signal applied to the baseband filter is inoperation from the main amplifier feedback loop.
 4. A device accordingto claim 1, wherein said main amplifier loop includes a down-mixer andbaseband filter and a phase shifter.
 5. A device according to claim 1,wherein said auxiliary loop includes an an AGC (automatic gain control)circuit arranged to receive in operation an portion of a signal which isapplied as an input signal to the upconverter and to apply an outputsignal to the adjustable driver, the auxiliary loop also including anadaptor arranged to control gain of the AGC circuit.
 6. A deviceaccording to claim 5, wherein said main amplifier loop includes adown-mixer and baseband filter and a phase shifter, wherein the mainamplifier feedback loop is operable to process a portion of an outputsignal of the amplifier by down-converting by said down-mixer andbaseband filter unit and phase shifting by said phase shifter, whereinsaid phase shifter is arranged to be controlled by a control signal fromsaid adaptor.
 7. A device according to claim 5, wherein said mainamplifier loop includes a down-mixer and a baseband filter and a phaseshifter and the device includes an attenuator arranged to provide anoutput signal which is combined with an input baseband signal to providean input to the baseband filter which in turn is arranged to provide anoutput signal to the upconverter, the attenuator being connected toreceive in operation an input signal component from the adaptor, anoutput of the attenuator being combined in operation prior toapplication to the baseband filter with an output signal of said mainamplifier feedback loop.
 8. A linear transmitter block of a radiocommunication device, said linear transmitter block comprising a mainamplifier closed feedback loop to control gain and linearity of a radiofrequency (RF) power amplifier and an auxiliary loop comprising anisolator eliminator to compensate at baseband for antenna loadvariations on the RF power amplifier, an upconverter for convertingbaseband signals into radio frequency signals, and a device forminimizing performance degradation of the linear transmitter block inthe presence of antenna interference, said device for minimizingperformance degradation comprising the RF power amplifier having anadjustable driver arranged to receive from said upconverter an inputsignal which includes a component from said main amplifier feedback loopand to receive a gain control signal from said auxiliary loop, saidamplifier being operable to amplify said received input signal accordingto said gain control signal and to provide an amplified output signal aportion of which is provided in operation to said main amplifierfeedback loop for generating the component included in the input signalfrom the upconverter.